A great effort has been made to develop technologies for cast molding of hydrogel contact lenses with high precision, fidelity and reproducibility and at low cost. One of such manufacturing technologies is the so-called Lightstream Technology™ (CIBA Vision) involving a lens-forming composition being substantially free of monomers and comprising a substantially purified prepolymer with ethylenically-unsaturated groups, reusable molds, and curing under a spatial limitation of actinic radiation (e.g., UV), as described in U.S. Pat. Nos. 5,508,317, 5,583,463, 5,789,464, and 5,849,810. The Lightstream Technology™ for making contact lenses have several advantages. First, the curing process is fast, at a time scale of seconds. Fast curing can ensure design and adaptation of a high speed, continuous and automatic lens production involving on-line lens curing. Second, by using a composition comprising a prepolymer and being substantially free of monomers, subsequent extraction steps (removing unpolymerized monomers from the lenses) required in a traditional cast-molding manufacturing process are eliminated. Without lens extraction, the production cost can be reduced and the production efficiency can be further enhanced. Third, reusable quartz/glass molds or reusable plastic molds, not disposable plastic molds, can be used, because, following the production of a lens, these molds can be cleaned rapidly and effectively of the uncrosslinked prepolymer and other residues, using a suitable solvent and can be blown dried with air. Disposable plastic molds inherently have variations in the dimensions, because, during injection-molding of plastic molds, fluctuations in the dimensions of molds can occur as a result of fluctuations in the production process (temperatures, pressures, material properties), and also because the resultant molds may undergo non-uniformly shrinking after the injection molding. These dimensional changes in the mold may lead to fluctuations in the parameters of contact lenses to be produced (peak refractive index, diameter, basic curve, central thickness etc.) and to a low fidelity in duplicating complex lens design. By using reusable molds which are produced in high precision, one can eliminate dimensional variations inherently presented in disposable molds and thereby variation in contact lenses produced therefrom. Lenses produced according to the Lightstream Technology™ can have high consistency and high fidelity to the original lens design.
However, there are some practical limitations which hinder realization of all of the great potentials of such technology. For example, a lens-forming composition may need to have relatively low viscosity so as to dispense the composition into molds at a high speed. To have a relatively low viscosity, a prepolymer in the composition may have to have a relatively lower molecular mass. It is believed that the molecular mass of a prepolymer may affect the mechanical strength of lenses made from crosslinking of the prepolymer. Lenses made from crosslinking of a prepolymer with a low molecular mass may not have a desired mechanical strength, such as, for example, low tearing resistance. Hydrogel contact lenses having low mechanical strength may not be suitable for daily- and extended-wear modality.
Accordingly, there is still a need for a lens manufacturing process for economically producing durable, highly elastic soft contact lenses with desired physical properties. There is also need for new actinically-crosslinkable prepolymers suitable for making hydrogel contact lenses with desired mechanical strength and desired physical properties.